An elevator dispatcher causes a particular elevator in a bank of elevator cars to be sent to a floor in response to a user pressing a hall button at that floor. Traditionally, a hall lantern will illuminate just prior to the opening of the car doors in order to inform the user as to which car will service his hall call.
The dispatcher assigns a car to a hall call according to a variety of elevator system parameters. It is possible for values of these system parameters to change between the time the hall call is registered and the time the hall call is serviced. Therefore, the dispatcher may reassign the hall call to other cars many times before the hall call is serviced. The user does not notice the reassignment because the hall lantern is lit only after these multiple reassignments have occurred and just before the car arrives at the floor.
According to a dispatching scheme called instantaneous car assignment (ICA), once a car has been assigned to a hall call, the assignment may not be changed. Unlike traditional elevator assignment techniques, ICA informs the user at the instant of first assignment (or shortly thereafter) as to which car will service his/her hall call. The benefit is that the user can be walking toward that particular car, of the bank of cars, which is going to serve him and be positioned and ready to enter that car when it arrives. A know-and-go time is the time from when a passenger knows which car is responding to his hall call to the time it takes him to go over to the car. Therefore, giving the user the opportunity to be in front of the car when it arrives requires that numerous reassignments of a hall call to different cars cannot take place. To the extent a dispatcher is spending time reassigning, the know-and-go time is used up.
The reason for allowing multiple reassignments in the past was to obtain the best assignment; concern over an initial optimum assignment was minimized in the past because there would always be reassignments possible and therefore the opportunity to correct for an initial assignment that had become less than optimum in light of subsequent events such as new hall calls and car calls. Under ICA, however, because there is little or no time for reassignments, the importance of a good initial assignment is increased.
The first uses of ICA were not as sensitive to this issue as they might have been. Relative System Response (RSR), taught in U.S. Pat. No. 4,363,381 "Relative System Response Elevator Call Assignments", is one scheme typically used with the expectation that multiple reassignments would be allowed. ICA was used in conjunction with RSR. This RSR/ICA scheme, therefore, fixed the first car to hall call assignment using RSR--a scheme for which the initial assignment did not account for future events (new hall calls and car calls) which would serve to degrade the quality of an initial assignment. The need for a better initial assignment remained after RSR/ICA.
The average registration is the time from when the hall call button is pressed to the time that the hall call is cancelled. This latter point in time varies with different elevator systems--for some, the hall call is cancelled when the car arrives at the floor and is leveling while for others the hall call is cancelled at a stop control point typically located where deceleration of the elevator begins as it nears the floor begins and a hall lantern is lit. Note that registration time is not equal to waiting time because not all passengers wait the same time and therefore we cannot easily measure the waiting time of all passengers.
The average registration time of an elevator system is a common metric for the performance of that system. However, a good average registration time can be deceptive, hiding an occasional, extremely long registration time among numerous, very short registration times. Engineers have discovered that there will usually be one hall call during a heavy two-way traffic scenario which waits a very long time (for example, 135 seconds). These long waits occur rather infrequently (for example, once or twice in one thousand hall calls). It has been observed that the associated hall calls have often been bypassed by at least one (usually several) car. These bypasses happen because the bypassing car was not the one assigned to the hall call at the time of the bypass. If the bypassing car had stopped for the hall call, then the very long registration time could have been reduced.
Customers have pointed out the need to reduce these very long registration times. By reducing the number of hall call bypasses, a dispatcher may reduce the longest registration time. At the same time, however, the average of all registration times may increase because special treatment to a long-waiting call is given at the expense of several other hall calls. In some markets, it is understood that the market place will accept a higher average registration time in favor of a lower maximum registration time.
FIG. 1 illustrates this maximum registration time dilemma and the failure of the prior art to address it. According to the prior art, car B is assigned a hall call at floor 7 while car B is heading in the down direction when a new hall call at floor 9 is registered, which hall call is as yet not assigned. Car A is also heading in the down direction but is farther from the hall call registered at floor 9 than car B. According to the prior art RSR scheme, car B will more than likely be assigned to the hall call at floor 9 because car B is closer than car A to floor 9. This is optimum for the person who registered the hall call at floor 9, but the person who registered the hall call at floor 7, to which car B is already committed, had been waiting for sixty seconds for a car already when the hall call at floor 9 was registered. The person at floor 9 has a very short wait, but the person at floor 7 who has already waited a long time, now waits even longer.